This invention pertains, generally, to devices for screening fluid suspensions, such as screens and separators, and the like, and in particular to screening devices used in processing wood pulp slurries in papermaking processes, and to novel backwash means therefor.
Screenplates, or screens of such devices tend to clog with suspended fiber and, as a consequence thereof, means are required for cleaning the screens of the clogging fibers. Typically, hydrofoils are used to generate a backwash wave or surge to effect the necessary cleaning. In this, of course, there must be relative motion between the screen and the hydrofoil. The hydrofoil is arranged in proximate adjacency to the screen surface, whether the screen is cylindrical, disc or planar-shaped, or frustoconical.
The hydrofoils used for the aforesaid purpose may take many forms, including airfoil sections, radial vanes, and drum-mounted devices such as blades, bumps and half-foils. They are characterized by the generation of two, successive pressure pulses, in the slurry, the first pulse having a pressure greater than the ambient pressure, and the second having a pressure lower than the ambient.
Known apparatuses and devices which employ hydrofoils are characterized by high relative velocity between the screenplate or screen and the hydrofoil, and a backwash flow surge which is limited by various factors, in particular: by cavitation during the negative pressure pulse, and by a high, positive pulse resulting in an accept flow surge which tends to reduce separation efficiency.
The foregoing illustrates limitations known to exist in present screening devices. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.